Myelin Lamellae Research Articles

Combined Pre- and Postnatal Ethanol Exposure in Rats Disturbs the Myelination of Optic Axons†

To analyse myelination and outgrowth of the optic axons in relation to the neuro-ophthalmological manifestations of ethanol (EtOH) abuse during pregnancy. An experimental model of chronic EtOH exposure was developed in rats and their offspring by subjecting the dams to a liquid diet (35% of the daily total calories as either EtOH or maltose-dextrose nutritional controls (Con). Eyeballs and optic nerves were obtained at key developmental stages and processed for morphologic, immunocytochemical and immunoblotting procedures, using alternatively antibodies against myelin basic protein (MBP) or neurofilament (NF) protein, and image analysing. A significant delay in onset of optic axons myelination, as well as a significant reduction in optic nerve size (P < 0.001), optic axons number (P < 0.001), myelinated axons density (P < 0.001), number of myelin lamellae linked to axon diameter (P < 0.001) and optic axon cross-sectional area (P < 0.001) were detected in the global morphometric assessment of the EtOH nerves with respect to the Con. Expression of MBP and NF was noticeably reduced in the EtOH optic nerves when compared with the Con. Disturbed myelination of optic axons, caused by EtOH abuse, strongly disrupts the optic nerve development and the establishment of definitive retinal and optic nerve targets, and subsequently the visual patterns.

THE AQUEOUS LAYERS WITHIN THE MYELIN SHEATH MODULATE THE MEMBRANE PROPERTIES OF SIMULATED HEREDITARY DEMYELINATING NEUROPATHIES

To expand our studies on the mechanisms underlying the clinical decline of the nerve excitability properties in patients with hereditary demyelinating neuropathies, the contribution of myelin sheath aqueous layers on multiple membrane properties of simulated fiber demyelinations is investigated. Three progressively greater degrees of internodal systematic demyelinations (two mild and one severe termed as ISD1, ISD2 and ISD3, respectively) without/with aqueous layers are simulated using our previous multi-layered model of human motor nerve fiber. The calculated multiple membrane excitability properties are as follows: potentials (intracellular action, electrotonic), strength-duration time constants, rheobasic currents and recovery cycles. They reflect the propagating, accommodative and adaptive processes in the fibers. The results show that all membrane properties, except for the strength-duration time constants and refractoriness, worsen when the myelin lamellae and their corresponding aqueous layers are uniformly reduced along the fiber length. The effect of the aqueous layers is significantly higher on the accommodative and adaptive processes than on the propagating processes in the fibers. Our multi-layered model better approximated some of the functional deficits documented for axons of patients with Charcot-Marie-Tooth disease type 1A. The study provides new and important information on the mechanisms underlying the pathophysiology of hereditary demyelinating neuropathies.

Adult Ceramide Synthase 2 (CERS2)-deficient Mice Exhibit Myelin Sheath Defects, Cerebellar Degeneration, and Hepatocarcinomas

(Dihydro)ceramide synthase 2 (cers2, formerly called lass2) is the most abundantly expressed member of the ceramide synthase gene family, which includes six isoforms in mice. CERS2 activity has been reported to be specific toward very long fatty acid residues (C22-C24). In order to study the biological role of CERS2, we have inactivated its coding region in transgenic mice using gene-trapped embryonic stem cells that express lacZ reporter DNA under control of the cers2 promoter. The resulting mice lack ceramide synthase activity toward C24:1 in the brain as well as the liver and show only very low activity toward C18:0-C22:0 in liver and reduced activity toward C22:0 residues in the brain. In addition, these mice exhibit strongly reduced levels of ceramide species with very long fatty acid residues (>or=C22) in the liver, kidney, and brain. From early adulthood on, myelin stainability is progressively lost, biochemically accompanied by about 50% loss of compacted myelin and 80% loss of myelin basic protein. Starting around 9 months, both the medullary tree and the internal granular layer of the cerebellum show significant signs of degeneration associated with the formation of microcysts. Predominantly in the peripheral nervous system, we observed vesiculation and multifocal detachment of the inner myelin lamellae in about 20% of the axons. Beyond 7 months, the CERS2-deficient mice developed hepatocarcinomas with local destruction of tissue architecture and discrete gaps in renal parenchyma. Our results indicate that CERS2 activity supports different biological functions: maintenance of myelin, stabilization of the cerebellar as well as renal histological architecture, and protection against hepatocarcinomas.

Neuroprotective Effect of Melatonin on Experimental Peripheral Nerve Injury: An Electron Microscopic and Biochemical Study

Melatonin is an important antioxidant agent with a protective role in the prevention of oxidative stress. We designed an experimental study which focused on the potential neuroprotective effect of melatonin on peripheral nerve injury. Sciatic nerve injury was induced in the mid thigh region of 30 male Wistar rats by clip compression. Melantonin was injected intraperitoneally in 15 of the 30 rats. Electron microscope and biochemical studies were performed to assess the potential beneficial effect of melatonin on peripheral nerve regeneration. Changes to cellular organelles, myelin lamellae and axons were studied. There was a significant difference between the melatonin and nerve injury groups. Rats treated with melatonin demonstrated significant structural protection of the myelin lamellae compared to the nerve injury group. Axonal shrinkage and myelin changes were not prominent histopathologically in melatonin-treated group. Biochemical analysis confirmed the neuroprotective effects of melatonin with significantly lower lipid peroxidation and myeloperoxidase activity measurements in the melatonin-treated group compared to the neural injury group. The results indicate that melatonin can improve neural healing. With its neuroprotective effect, as demonstrated in this experimental peripheral nerve injury, melatonin might be used successfully in clinical practice. Further studies on the correct dosage and possible side effects are necessary.

Phase Transition Behaviors And Interactions And Adhesion Of Myelin Lipid Membranes Modulated By Myelin Basic Protein

Myelin is a stacked membrane structure that allows for fast, efficient conduction of nerve impulses. It has 8 kinds of lipid molecules on two alternating bilayers and proteins such as myelin basic protein (MBP) which has an important role in maintaining myelin structure. The compact bilayer organization of healthy myelin is believed to require a well-defined range of lipid and protein composition, and lipid-protein interaction. Even though we know that multiple sclerosis (MS) is a morphological transformation involving loss of adhesion between myelin lamellae and sometimes formation of myelin vesicle, its mechanism and causes for demyelination are still under investigation.We have used fluorescence microscopy, Langmuir isotherm, and Langmuir-Blodgett (LB) techniques to investigate how lipid composition of myelin lipid system affects the phase transition behaviors of myelin monolayers and bilayers depending on lateral pressure, temperature, and pH conditions. Model membranes with the composition of the cytoplasmic side of experimental allergic encephalomyelitis (EAE) myelin were also constructed on mica surfaces by LB deposition and the forces between the surfaces measured using the Surface Forces Apparatus (SFA) after exposure to various solution concentrations of MBP.Our findings clearly demonstrate EAE monolayer remains phase-separated under physiological conditions. If the myelin sheath were to form two phases in vivo there are a variety of effects that could result. The line tension between two segregated domains and a local repulsive force could cause the membrane to bulge leading to vesiculation of the membrane. Force-distance measurements between supported myelin bilayers mimicking the cytoplasmic surface of myelin at various surface coverages of MBP indicate that maximum adhesion and minimum cytoplasmic spacing occurs when each negative lipid in the membrane can bind to a positive arginine or lysine group on MBP.

Diffusion Weighted Imaging in Heroin-associated Spongiform Leukoencephalopathy

Heroin-associated spongiform leukoencephalopathy (HASL) is a rare complication of heroin abuse. We report a case that highlights the increased sensitivity of diffusion weighted imaging (DWI) compared with T2-weighted imaging in the acute setting of HASL. Single case report. A 36-year-old male with a history of heroin abuse (snorting) presented with a 3-day history of lethargy. MRI brain revealed restricted diffusion involving the globus pallidum and cerebral cortex bilaterally that was not seen on fluid-attenuated inversion recovery (FLAIR) images. The patient was diagnosed with acute HASL. Repeated MRI FLAIR at 3 months confirmed the development of atrophy and T2 hyperintensity in the subcortical white matter, consistent with leukoencephalopathy. Neurological exam at 3-month follow-up was nonfocal. Restricted diffusion, which likely corresponds to electron microscopic findings of fluid entrapment between the myelin lamellae, may be detectable earlier than changes on FLAIR. Clinicians should be aware of the neuroimaging findings of HASL and the increased sensitivity of MRI DWI over T2-weighted images in detecting HASL acutely.

Effects of osmolality on PLP-null myelin structure: Implications re axon damage

In order to test the adhesiveness of PLP-null compact myelin lamellae we soaked aldehyde-fixed CNS specimens from PLP-null and control mice overnight in distilled water, in Ringer's solution or in Ringer's solution with added 1 M sucrose. Subsequent examination of the tissue by EM showed that both PLP-null and control white matter soaked in Ringer remained largely compact. After the distilled water soak, control myelin was virtually unchanged, but PLP-null myelin showed some decompaction, i.e., separation of myelin lamellae from one another. After the sucrose/Ringer soak, normal myelin developed foci of decompaction, but the great majority of lamellae remained compact. In the PLP-null specimens, in contrast, many of the myelin sheaths became almost completely decompacted. Such sheaths became thicker overall and were comprised of lamellae widely separated from one another by irregular spaces. Thus, in normal animals, fixed CNS myelin lamellae are firmly adherent and resist separation; PLP-null myelin lamellae, in contrast, are poorly adherent and more readily separated. Mechanisms by which impaired adhesiveness of PLP-null myelin lamellae and fluctuations in osmolality in vivo might underlie slowing of conduction and axon damage are discussed.

Reflections

Reflections

Spinal cord dysmyelination caused by an antiproteolipid protein IgM antibody: Implications for the mechanism of central nervous system myelin formation

Antiglycolipid IgM antibodies are known to induce formation of "wide spaced" or "expanded" myelin, a distinctive form of dysmyelination characterized by a repeat period approximately two or three times normal, which is seen also in diseases, including multiple sclerosis. To determine whether an antibody directed against a myelin protein would cause equivalent pathology, we implanted O10 hybridoma cells into the spinal cord of adult or juvenile rats. O10 produces an IgM directed against PLP, the major protein of CNS myelin. Subsequent examination of the cords showed focal demyelination and remyelination. In addition, however, some juvenile cords, but none of the adult cords, displayed wide-spaced myelin with lamellae separated by an extracellular material comprising elements consistent with IgM molecules in appearance. Wide spacing tended to involve the outer layers of the sheath and in some cases alternated with normally spaced lamellae. A feature not seen previously consists of multiple expanded myelin lamellae in one sector of a sheath continuous with normally spaced lamellae in another, resulting in variation in sheath thickness around the axonal circumference. This uneven distribution of wide-spaced lamellae is most simply explained based on incorporation of IgM molecules into immature sheaths during myelin formation and implies a model of CNS myelinogenesis more complex than simple spiraling. The periaxonal space never displays widening of this kind, but the interface with adjacent myelin sheaths or oligodendrocytes may. Thus, wide spacing appears to require that IgM molecules bridge between two PLP-containing membranes and does not reflect the mere presence of immunoglobulin within the extracellular space.

Neuropathic pain correlates with myelinated fibre loss and cytokine profile in POEMS syndrome

Objective:To reveal characteristic clinicopathological correlates of polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy and skin changes (POEMS) syndrome.Methods:The clinical features of 22 patients with POEMS syndrome were investigated and correlated with the...

Neuroprotection and enhancement of remyelination by estradiol and dexamethasone in cocultures of rat DRG neurons and Schwann cells

A simple and reproducible demyelination–remyelination system was developed with cocultures of rat dorsal root ganglia (DRG) neurons and Schwann cells, and the effects of steroid hormones were examined in this system. Addition of forskolin to the cocultures induced demyelination and lower levels of myelin protein P0 expression were seen in immunoblots after eight days of treatment. Removal of forskolin caused remyelination and the amount of P0 expression was partially recovered. States of demyelination and remyelination were further confirmed by morphological examination of myelin internodes, such as incorporation of fluorescently-labeled fatty acid and immunostaining of P0 and myelin associated glycoprotein. Ultrastructural analyses of the cocultures showed the presence of myeloid bodies and peeling of myelin lamellae during the demyelination process. Using this demyelination–remyelination system, regulatory roles of steroid hormones during demyelination and remyelination were studied. Immunoblots of P0 and incorporation of fluorescently-labeled fatty acid demonstrated that treatments of the cocultures with dexamethasone and estradiol not only reduced the extent of demyelination but also enhanced remyelination. These results suggest that steroid hormones have roles as neuroprotective agents against demyelination and augmentative agents for remyelination.

Developmental loss of NT‐3 in vivo results in reduced levels of myelin‐specific proteins, a reduced extent of myelination and increased apoptosis of Schwann cells

This work investigates the role of NT-3 in peripheral myelination. Recent articles, based in vitro, propose that NT-3 acting through its high-affinity receptor TrkC may act to inhibit myelin formation by enhancing Schwann cell motility and/or migration. Here, we investigate this hypothesis in vivo by examining myelination formation in NT-3 mutant mice. On the day of birth, soon after the onset of myelination, axons showed normal ensheathment by Schwann cells, no change in the proportion of axons which had begun to myelinate, and no change in either myelin thickness or number of myelin lamellae. However in postnatal day 21 mice, when myelination is substantially complete, we observed an unexpected reduction in mRNA and protein levels for MAG and P(0), and in myelin thickness. This is the opposite result to that predicted from previous in vitro studies, where removal of an inhibitory NT-3 signal would have been expected to enhance myelination. These results suggest that, in vivo, the importance of NT-3 as a major support factor for Schwann cells (Meier et al., (1999) J Neurosci 19:3847-3859) over-rides its potential role as an myelin inhibitor, with the net effect that loss of NT-3 results in degradation of Schwann cell functions, including myelination. In support of this idea, Schwann cells of NT-3 null mutants showed increased expression of activated caspase-3. Finally, we observed significant reduction in width of the Schwann cell periaxonal collar in NT-3 mutant animals suggesting that loss of NT-3 and resulting reduction in MAG levels may alter signaling at the axon-glial interface.

Diagnostic value of ultrastructural nerve examination in Charcot-Marie-Tooth disease: two CMT 1B cases with pseudo-recessive inheritance

We report two sporadic patients of CMT disease in different consanguineous families. The electrophysiological examination led to the diagnosis of a severe demyelinating neuropathy. The nerve biopsies exhibited numerous outfoldings of the myelin sheaths and onion-bulb proliferations. The consanguinity and the histological findings pointed to a diagnosis of CMT 4B. However, the detection of abnormal and regular widenings between the major dense lines of the myelin lamellae by electron microscopy led us to search for a P0 gene mutation. Two heterozygous mutations of this gene were identified: S63F and N131Y. Different aspects of uncompacted myelin lamellae have been described in some cases of P0 mutations and a few now appear to be quite specific to it. More than 30 genes are implicated in CMT and as mutation search is time- and money-consuming, we believe that in some selected patients ultrastructural examination of nerves, among other criteria, helps orientate the molecular diagnosis of CMT.

Myelin Vacuolation, Optic Neuropathy and Retinal Degeneration after Closantel Overdosage in Sheep and in a Goat

Toxicity of closantel, a halogenated salicylanilide anthelmintic, is described in 11 sheep and a goat, humanely killed 4-70 days after accidental overdosage. Status spongiosis of the cerebrum and cerebellum was present, its severity decreasing with time after treatment. Ultrastructurally, vacuoles in the cerebral white matter were seen to be intramyelinic due to splitting of myelin lamellae at the intraperiod lines, indicating myelin oedema. In the optic nerves, Wallerian degeneration and eventual fibrosis and atrophy of the nerves followed myelin vacuolation. Lesions in the optic nerves were particularly advanced in the intracanalicular portion, indicating a compressive neuropathy within the optic canal. Acute retinal lesions consisted of papilloedema, necrosis of the outer retinal layers (especially the photoreceptor layer), and retinal separation in tapetal and non-tapetal areas. In more chronic cases, the outer nuclear layer was diffusely attenuated and generally reduced to a single row of cells.

Electrophysiological correlates of progressive sensorineural pathology in carboplatin-treated chinchillas

Carboplatin produces progressive damage to auditory nerve fibers, spiral ganglion neurons (SGNs) and inner hair cells (IHC) in the chinchilla cochlea but leaves outer hair cells intact. Within 1 h after injection, many afferent terminals beneath IHCs and myelin lamellae surrounding SGN processes are vacuolated. One day after injection, approximately half of the nerve fibers are missing. IHCs are intact at 2 days, but 20–30% are missing at 3 days. We studied the electrophysiological correlates of this progressive morphological damage by recording cochlear microphonics (CM), distortion product otoacoustic emissions (DPOAE), summating potentials (SP), compound action potentials (CAP) and midbrain evoked potentials (IC-EVP) before and 1 h, 12 h, 1 days, 3 days, 5 days, 7 days and 14 days after carboplatin injection (75 mg/kg IP) in four chinchillas. CM and DPOAEs tended to be unchanged or enhanced. CAP and SP showed little change until Day 3, when amplitudes were reduced in all animals and CAP thresholds were elevated by 9 dB; amplitudes declined further between Days 3 and 5 but not thereafter. IC-EVP amplitudes decreased on Days 3 or 5 but thresholds were relatively unchanged. All animals showed some recovery of IC-EVP between Days 7 and 14, including one with 70% enhancement on Day 14. The results indicate that threshold and amplitude measures fail to detect peripheral pathology until some relatively high threshold level of damage has been exceeded. This has important implications for monitoring peripheral damage and interpreting electrophysiological test results in animals and humans.

SIMULATING MILD SYSTEMATIC AND FOCAL DEMYELINATING NEUROPATHIES: MEMBRANE PROPERTY ABNORMALITIES

This study provides numerical simulations of some of the abnormalities in the potentials and axonal excitability indices of human motor nerve fibers in simulated cases of internodal, paranodal and simultaneously of paranodal internodal demyelinations, each of them systematic or focal. A 70% reduction of the myelin lamellae (defining internodal demyelination), or of the paranodal seal resistance (defining paranodal demyelination), or simultaneously both of them (defining paranodal internodal demyelination) was uniform along the fiber length for the systematically demyelinated subtypes. These permutations were termed internodal systematic demyelination (ISD), paranodal systematic demyelination (PSD) and paranodal internodal systematic demyelination (PISD). In other tests, the same reductions of the myelin sheath parameters were used but restricted to only three (8th, 9th and 10th) consecutive internodes. Such fiber demyelinations were termed internodal focal demyelination (IFD), paranodal focal demyelination (PFD) and paranodal internodal focal demyelination (PIFD). The computations used our previous double cable model of the fibers. The axon model was comprised of 30 nodes and 29 internodes. The 70% reduction value was not sufficient to develop conduction block in all investigated demyelinations, which were regarded as mild. The membrane property abnormalities obtained in the ISD, PSD and PISD cases were quite different and abnormally greater than those in the IFD, PFD and PIFD cases. The changes in the excitability indices such as strength-duration time constants, rheobasic currents and recovery cycles in the focally demyelinated subtypes were so slight as to be essentially indistinguishable from normal values. Consequently, the excitability based approaches that have shown strong potential as diagnostic tools in systematically demyelinated conditions may not be useful in detecting mild focal demyelinations. The membrane property changes simulated in the systematically demyelinated subtypes are in good accordance with the data from patients with Charcot-Marie-Tooth disease type 1A (CMT1A) and chronic inflammatory demyelinating polyneuropathy (CIDP). The excitability abnormalities obtained in each focally demyelinated subtype match those observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome (GBS). The results indicate that the model that was used is a rather promising tool in studying the membrane property abnormalities of hereditary, chronic and acquired demyelinating neuropathies, which up till now, have not been sufficiently well understood.

Simulating focal demyelinating neuropathies: membrane property abnormalities

Membrane properties such as potentials (intracellular, extracellular, electrotonic) and axonal excitability indices (strength-duration and charge-duration curves, strength-duration time constants, rheobasic currents, recovery cycles) can now be measured in healthy subjects and patients with demyelinating neuropathies. They are regarded here in two cases of simultaneously reduced paranodal seal resistance and myelin lamellae in one to three consecutive internodes of human motor nerve fiber. The investigations are performed for 70 and 96% myelin reduction values. The first value is not sufficient to develop a conduction block, but the second leads to a block and the corresponding demyelinations are regarded as mild and severe. For both the mild and severe demyelinations, the paranodally internodally focally demyelinated cases (termed as PIFD1, PIFD2, and PIFD3, respectively, with one, two, and three demyelinated internodes) are simulated using our previous double-cable model of the fiber. The axon model consists of 30 nodes and 29 internodes. The membrane property abnormalities obtained can be observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome (GBS) and multifocal motor neuropathy (MMN). The study confirms that focal demyelinations are specific indicators for acquired demyelinating neuropathies. Moreover, the following changes have been calculated in our previous papers: (1) uniform reduction of myelin thickness in all internodes (Stephanova et al. in Clin Neurophysiol 116: 1153-1158, 2005); (2) demyelination of all paranodal regions (Stephanova and Daskalova in Clin Neurophysiol 116: 1159-1166, 2005a); (3) simultaneous reduction of myelin thickness and paranodal demyelination in all internodes (Stephanova and Daskalova in Clin Neurophysiol 116: 2334-2341, 2005b); and (4) reduction of myelin thickness of up to three internodes (Stephanova et al., in J Biol Phys, 2006a,b, DOI: 10.1007/s10867-005-9001-9; DOI: 10.1007/s10867-006-9008-x). The membrane property abnormalities obtained in the homogeneously demyelinated cases are quite different and abnormally greater than those in the case investigated here of simultaneous reduction in myelin thickness and paranodal demyelination of up to three internodes. Our previous and present results show that unless focal demyelination is severe enough to cause outright conduction block, changes are so slight as to be essentially indistinguishable from normal values. Consequently, the excitability-based approaches that have shown strong potential as diagnostic tools in systematically demyelinated conditions may not be useful in detecting mild focal demyelinations, independently of whether they are internodal, paranodal, or paranodal internodal.

N.P.3 08 Contribution of the nerve biopsy in hereditary peripheral neuropathies

Most cases of hereditary peripheral neuropathies (HPN) are now diagnosed by molecular biology, but the peripheral nerve biopsy (PNB) is still contributory in a few atypical cases. On the basis of a 30-year-experience of PNB, we wish to illustrate histopathological features which may inform the molecular biologist toward a specific genetic abnormality. A whole nerve biopsy is performed on the superficial peroneal or the sural nerve. Each case is prepared for light and electron microscopy (EM). On paraffin sections, small amyloid deposits scattered in the endoneurium, or intra-axonal polyglucosan bodies, can occasionally be disclosed. Semi-thin sections sometimes exhibit features of hypermyelination, i.e. tomaculae, or onion-bulb formations (OBF) indicating previous episodes of primary demyelination. EM is more informative, as it reveals nerve fiber modifications characteristic of certain types of HPN. Tomaculae correspond to hereditary neuropathy to pressure palsy, which presents in certain cases as acute symmetrical polyneuropathy, but tomaculae can also be the prominent feature in certain cases of Charcot-Marie-Tooth (CMT) disease type 1b. Other CMT1b families exhibit mainly features of uncompacted myelin lamellae. Myelin outfoldings are rarely observed and point to a recessive form, CMT4. Prominent OBF are present in every case of CMT1a and also in rare cases of chronic inflammatory demyelinating polyneuropathy (CIDP). Atypical OBF, mainly made of thin remnants of basal lamina are only observed in CMT infantile cases, so-called Dejerine-Sottas disease. Intra-axonal filament accumulation, with sometimes giant axons are seen in recessive cases, CMT4. Macrophage-associated demyelination is mainly observed in CIDP cases, but can reveal genuine cases of CMT1a presenting with a concurrent auto-immune inflammatory process. Occasionally, EM can disclose storage material in ceroid lipofuscinosis, adrenoleukodystrophy or Tangier’s disease. PNB can reveal familial pathology in pseudo-sporadic cases of any age and orientate molecular genetics investigation in rare cases.

Subtle myelin defects in PLP‐null mice

This study explores subtle defects in the myelin of proteolipid protein (PLP)-null mice that could potentially underlie the functional losses and axon damage known to occur in this mutant and in myelin diseases including multiple sclerosis. We have compared PLP-null central nervous system (CNS) myelin with normal myelin using ultrastructural methods designed to emphasize fine differences. In the PLP-null CNS, axons large enough to be myelinated often lack myelin entirely or are surrounded by abnormally thin sheaths. Short stretches of cytoplasm persist in many myelin lamellae. Most strikingly, compaction is incomplete in this mutant as shown by the widespread presence of patent interlamellar spaces of variable width that can be labeled with ferricyanide, acting as an aqueous extracellular tracer. In thinly myelinated fibers, interlamellar spaces are filled across the full width of the sheaths. In thick myelin sheaths, they appear filled irregularly but diffusely. These patent spaces constitute a spiral pathway through which ions and other extracellular agents may penetrate gradually, possibly contributing to the axon damage known to occur in this mutant, especially in thinly myelinated fibers, where the spiral path length is shortest and most consistently labeled. We show also that the "radial component" of myelin is distorted in the mutant ("diagonal component"), extending across the sheaths at 45 degrees instead of 90 degrees. These observations indicate a direct or indirect role for PLP in maintaining myelin compaction along the external surfaces of the lamellae and to a limited extent, along the cytoplasmic surfaces as well and also in maintaining the normal alignment of the radial component.

Differences in membrane properties in simulated cases of demyelinating neuropathies: internodal focal demyelinations with conduction block.

The aim of this study is to investigate the membrane properties (potentials and axonal excitability indices) in the case of myelin wrap reduction (96%) in one, two and three consecutive internodes along the length of human motor nerve fibre. The internodally focally demyelinated cases (termed as IFD1, IFD2 and IFD3, respectively, with one, two and three demyelinated internodes are simulated using our previous double cable model of the fibre. The progressively greater increase of focal loss of myelin lamellae blocks the invasion of the intracellular potentials into the demyelinated zones. For all investigated cases, the radial decline of the extracellular potential amplitudes increases with the increase of the radial distance and demyelination, whereas the electrotonic potentials show a decrease in the slow part of the depolarizing and hyperpolarizing responses. The time constants are shorter and the rheobases higher for the IFD2 and IFD3 cases than for the normal case. In the recovery cycles, the same cases have less refractoriness, greater supernormality and less late subnormality than the normal case. The simulated membrane abnormalities can be observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome. The study provides new information about the pathophysiology of acquired demyelinating neuropathies.